Steel is galvanized by passing a continuous strip around a roll submerged in a bath of molten zinc or aluminum/zinc. The roll assembly includes a roll, a pair of holding arms and a pair of trunnions attached to the end of the roll to guide and provide tension to the metal strip being galvanized.
A cylindrical surface on the roll-bearing components rotates in a cylindrical surface or opening in the arm-bearing components. The coating lines operate at very low speed, and because of distortions of the arms and the roll due to stresses and high temperature exposure, a large clearance exists between the rotating and the stationary bearing components.
Consequently, only a single line, metal-to-metal contact exists between the two bearing surfaces, creating a very high surface unit loading that produces a metal-to-metal or boundary lubrication area, thus resulting in high wear, micro welding and other severe wear conditions. The large clearances further decrease the possibility of forming or creating hydrodynamic film lubrication and in addition, allow large contaminants to penetrate the sliding bearing areas, destroying the surface finish and ultimately resulting in the bearing""s failure.
Summarizing: Galvanizing roll assemblies fail primarily due to premature bearing failures. The major reasons for bearing failures are:
a) Sliding friction: Creates extremely high coefficient of friction and micro-welding;
b) Single line contact: Creates extremely high surface unit loading,   P  =            F      A        =          Load      BearigArea      
xe2x80x83Where F=roll weight+strip tension=8,000 to 12,000 pounds
A=bearing contact surface area≅0 at running start;
c) High material solubility and low hardness: Creates micro-welding, corrosive wear, abrasive wear;
d) Low operating speed: The hydrodynamic lubrication coefficient defined by:       L    c    =            μ      ⁢              xe2x80x83            ⁢      N        P  
xe2x80x83Where:
Lc=hydrodynamic lubrication coefficient
xcexc=fluid viscocity
N=roll speed (revolutions per minute)
P=defined before in item (b)
Creates very low Lc values which translates into very high friction coefficients by lack of formation of hydrodynamic film lubrication, accelerating bearing failure
e) Large clearances, 0.350 in+: The strength of a hydrodynamic film as well as its formation and operating zone are inversely proportional to the bearing clearances. Large clearances eliminate any type of lubrication possibility.
I have solved some problems related to the environmental conditions that limit the life of apparatus submerged in molten metal. See, for example, my U.S. Pat. No. 5,549,393 issued Aug. 27, 1996, for xe2x80x9cSelf-Aligning Bearing for High Temperature Applicationsxe2x80x9d; U.S. Pat. No. 5,718,517 issued Feb. 17, 1998, for xe2x80x9cSelf-Aligning Bearing for High Temperature Applications; U.S. Pat. No. 6,261,369 issued Jul. 17, 2001 for xe2x80x9cSink Roll for Galvanizing Bathxe2x80x9d; U.S. Pat. No. 6,004,507 issued Dec. 21, 1999, for xe2x80x9cMaterial Formulation for Galvanizing Equipment Submerged in Molten Zinc and Aluminum/Zinc Meltsxe2x80x9d; and U.S. Pat. No. 6,168,757 issued Jan. 2, 2001 for xe2x80x9cMaterial Formulation for Galvanizing Equipment Submerged in Molten Aluminum and Aluminum/Zinc Meltsxe2x80x9d.
However, a need still exists for a bearing having a longer life when submerged in molten metal. One approach is to use materials that resist solubility in galvanizing materials such as molten zinc or zinc aluminum alloys (Item xe2x80x9ccxe2x80x9d in above failure list). See for example my U.S. Pat. No. 6,168,757 issued Jan. 2, 2001, for Material Formulation for Galvanizing Equipment Submerged in Molten Aluminum and Aluminum/Zinc Melts which is incorporated herein by reference.
Another approach to reducing low speed sliding friction caused by a single line, metal-to-metal bearing contact is to use a single line ceramic-to-ceramic contact. Such an approach is disclosed in Patent Application DE19511943A1 laid open by the German Patent Office Oct. 10, 1996. This patent was issued to Heinrich Pennenbecker. Pennenbecker discloses a trunnion mounted in a bearing housing supported by roll-holding arms. The trunnion has a series of circumferentially spaced, axially extending recesses. An elongated ceramic bearing box is mounted in each recess. Each bearing box extends beyond the trunnion surface, slidably engaging the cylindrical opening of the bearing housing.
Pennenbecker""s bearing does not address all the bearing failure reasons of the conventional galvanizing bearings which employ a shaft with a single line contact but, in addition, he has introduced some problems of his own. The difference in the coefficients of expansion between the steel housing and the ceramic bearing shell creates a large clearance at galvanizing operating temperature (900xc2x0 F.). It can be demonstrated that shrink-fitting of the shell is not possible nor sufficient to solve the problem.
Further, the inserts have to be made square to prevent rotation and breakage, eliminating multiple point contact and making hydrodynamic lubrication impossible. Ceramic material is brittle, which makes it very sensitive to bending, impact stresses, and contamination, especially with large bearing clearances. Ceramic-metal bearings as well as ceramic-ceramic bearings tend to fail suddenly (catastrophic failure) due to their intrinsic brittleness, while super alloy bearings wear out slowly giving the operator either time or warning of their need for replacement. Ceramic inserts tend to break with minor misalignments or impacts because of the roll""s weight, which may be more than two tons, added to a strip tension of 6,000 to 10,000 pounds. Overloading unlubricated ceramic material causes dynamic instability.
Although Pennenbecker has been part of the prior art for several years, I am not aware of anyone using these bearings. My new design addresses all points in the bearing failure list (xe2x80x9caxe2x80x9d thru xe2x80x9cexe2x80x9d).
My new design contemplates at least three roller-bearing embodiments. In one embodiment, the rollers are contained within a cage between the trunnion and the bearing shell. Rollers made with materials disclosed in my prior patents are suitable for most loads (Item xe2x80x9ccxe2x80x9d in failure list).
The preferred embodiment employs a plurality of rollers, eliminating 95% of the sliding friction by introducing xe2x80x9crolling motionxe2x80x9d (Item xe2x80x9caxe2x80x9d in failure list) and multiple line contact (Item xe2x80x9cbxe2x80x9d) in lieu of single line contact. The number of rollers is increased with increasing loads, to assure a larger number of line contacts. Mounted in a cradle sleeve, the rollers are between a trunnion having a cylindrical outer surface and a self-aligning shell (bearing housing) having a cylindrical inner surface (outer race) around the rollers. In one embodiment, the trunnion is attached to the end of the roll and rotates on the rollers located within a rocking bearing shell contained in the holding arm. In the other embodiment, the rollers are mounted between a trunnion fixed to the holding arm, and a self-aligning roller carrying bearing shell driven by the roll.
In both embodiments, each roller is partially but rotatably contained within a cylindrical recess in a cradle. In either case, each roller is mounted in its recess with the necessary clearance to form a hydrodynamic lubricating film (Item e in failure list) between the concave surface of the recess and the surface of the roller. The roller speed NR (RPM) is several times the rolls"" trunnion speed because the roller diameter is much smaller than the trunnion diameter:       N    R    =                    N        T            ⁢              D        T                    D      R      
NR=Roller RPM
DT=Trunnion diameter
DR=Roller diameter
NT=Trunnion RPM=roll RPM
Providing NR greater than 5NT (Item xe2x80x9cdxe2x80x9d in failure list)
This advantage when combined with the reduction of surface unit loading by multiple line contact, gives my design a lubrication coefficient Lc 15 to 25 times higher than a standard state-of-the-art bearing.
In addition, this arrangement provides a special advantage because a conventional roller-bearing typically has two points of contact, one with a non-rotating element and the other with a rotating element. In each case, the element having the larger bearing diameter will cause the roller to slide part of its travel motion, because the radius of the outer race is greater than the radius of the inner race.
The hydrodynamic film formed, which is described in some detail in my prior co-pending application, eliminates sliding friction between the roller and the cage in which the roller is mounted, while providing a high loading capacity and multiple contact areas. The bearing is preferably self-aligning by using a rocking sleeve-bearing race combination. The rocking sleeve permits a swiveling relationship to compensate for any misalignment between the axis of rotation of the trunnion within either the arm or the roll. The bearing races minimize the friction areas while providing for contamination xe2x80x9ccleaningxe2x80x9d (wiping) by utilizing high hardness/low friction coefficient super alloy materials.
Still further objects and advantages of the invention will become readily apparent to those skilled in the art to which the invention pertains upon reference to the following detailed description.